January 2006

BackgroundOver the last 40 years, a great deal of information related to damage of public works facilities has been collected and documented by the American Society of Civil Engineers (ASCE) Technical Council on Lifeline Earthquake Engineering (TCLEE). This includes data on all "lifelines" such as water systems, sewer systems, seaports, airports, electrical systems, telecommunication systems, gas and liquid fuel systems, hospitals and emergency response. This report will highlight some of the more relevant conditions related to water, wastewater and roads.

Earthquake Hazards - Soil InteractionsStrong shaking during an earthquake creates surface movements which cause obvious damage to unanchored above-grade facilities. However, what seems to cause even greater damage are the soil conditions that the utilities are constructed on. Depending on conditions, ground shaking can be amplified by "soil effects" increasing damage over those utilities constructed on different soil types.

Liquefaction of soils results in the soil losing its bearing capacity causing movement and foundation support failures for any at-grade or buried structures. This condition can also cause subsidence and lateral spreading. In addition, earthquake shaking can cause landslides in unstable areas. Utilities on or in the path of these earth movements will be impacted. Lastly, faulting or surface cracking directly in the fault zone can cause significant damage to ridged facilities subjected to vertical and horizontal displacements. Design and construction of public works facilities in earthquake zones need to consider both the strong motion of the earthquake and the secondary effects of site conditions which can increase the vulnerability of those facilities.

Three-foot horizontal offset caused extensive pipeline repairs.

Water/Sewer System VulnerabilitiesFor buried pipelines, soil conditions govern. Pipelines constructed in stable soils are only subjected to the strong shaking. This can result in some pipe movement in the trench causing joint compression, lateral offsets and increased joint stresses. For fragile pipelines, such as PVC, these additional stresses can be enough to cause the pipeline to crack. For those pipelines placed in areas subject to faulting (surface cracking due to earth movement), lateral spreading, liquefaction and landslides, the vulnerabilities are greatly increased. These movements can cause pipeline joints to separate, pipes to shear, and otherwise render the piping systems unusable. In areas of high groundwater, liquefaction in areas with gravity sewers has caused the pipelines and manholes to float to grades that cannot be readily restored.

Pipeline interfaces with other structures are also weak points during strong motion events. Pipes entering structures, such as tanks and treatment plant basins, can shear due to differential movement between the large structure and the smaller pipeline. Rigid pipeline interfaces at bridge abutments and hanger-supported pipelines under bridges have also been subject to failure.

Water and sewer facilities also have tanks and reservoirs that contact large quantities of water. This water, when subjected to earthquake movements, can also move generating great forces against the containment structures and typically fragile equipment (i.e., weirs, baffles, instrumentation). These strong movements can also cause above-grade tanks to move, severing connecting piping.

In addition to bulk water movement, multiple failures in pressure water systems (pipe breaks and unanticipated pump shutdowns) can result in pressure surges (water hammer) that can affect appurtenances that otherwise were not impacted by the earthquake.

Most water and sewer systems have treatment plants, pumps and other electrically operated equipment that is especially critical after an earthquake. Most earthquakes cause immediate power outages that can last many days. If there is no emergency power available, critical facilities cannot operate impacting many people and businesses. Even if emergency power is available, quite often they are never subjected to fully-loaded operation for extended periods. As a result, generators often fail when they are most needed. Providing refueling of the generators has also proven to be difficult after an earthquake directly impacting extended generator performance.

In addition to the obvious vulnerabilities, facilities that were constructed under previous codes can be vulnerable to damage during design earthquake events.

Emergency Response InteractionsMany critical lifeline utilities share common utility corridors. As a result, damage to one utility can cause collateral damage to one or more others. As such, repair and response times can be increased. In addition, emergency responders for one facility may not know of the damage to another and actually hinder recovery efforts. Some typical interactions are shown below:

Curt Edwards, P.E., is a Vice President at Psomas in the company's San Diego office. He is a member of APWA's Emergency Management Committee and is the Chairman of the ASCE Technical Council on Lifeline Earthquake Engineering, Earthquake Investigation Committee. He can be reached at (858) 576-9200 or cedwards@psomas.com.

Psomas is a leading consulting engineering firm offering services in water resources, planning and entitlements, land surveying and mapping, construction management, environmental, geographic information systems, land development engineering, and transportation to private sector, municipal/local agency, state and regional agency and federal agency clients.